Formaldehyde \For*mal"de*hyde\, n. [Formic + aldehyde.] (Chem.)
   A colorless, volatile liquid, {H2CO}, resembling acetic or
   ethyl aldehyde, and chemically intermediate between methyl
   alcohol and formic acid.
   
[1913 Webster]

Aldehyde \Al"de*hyde\ ([a^]l"d[-e]*h[imac]d), n. [Abbrev. fr.
   alcohol dehydrogenatum, alcohol deprived of its hydrogen.]
   1. (Chem.) A colorless, mobile, and very volatile liquid
      obtained from alcohol by certain processes of oxidation.
      
[1913 Webster]


   2. (Chem.) Any compound having the group {-CHO}. Methyl
      aldehyde, the simplest aldehyde, is more commonly called
      formaldehyde, {H-CHO}, and acetic aldehyde is now more
      commonly called {acetaldehyde}. The higher aldehydes may
      be solids. A {reducing sugar} typically contains the
      aldehyde group.
      [PJC]

   Note: The aldehydes are intermediate between the alcohols and
         acids, and differ from the alcohols in having two less
         hydrogen atoms in the molecule, as common aldehyde
         (called also {acetaldehyde}, {acetic aldehyde} or
         {ethyl aldehyde}), {C2H4O}; methyl aldehyde (called
         also {formaldehyde}), {CH2O}.
         [1913 Webster +PJC]

   {Aldehyde ammonia} (Chem.), a compound formed by the union of
      aldehyde with ammonia.
      
[1913 Webster]